In a case where a steam turbine in a stop state is put into operation, a warm-up operation is performed with rotation below a rated speed because a rotor is distorted due to its own weight during the long-term stop and the distortion has to be corrected.
As illustrated in FIG. 10, a multi-valve-type steam valve 106 that is provided with a plurality of control valves is used in the warm-up operation. An inner bar 13 that is disposed in a steam chamber to which steam is supplied and a plurality of control valves 114 that are mounted on the inner bar 13 constitute the multi-valve-type steam valve 106. The control valves 114 are driven in a vertical direction when the inner bar 13 is operated by supporting rods 17. The plurality of control valves 114 are set to reach an open state in order when the inner bar 13 is moved upward.